I can’t believe no one has suggested this: Take the scale you have at home, and move it to another part of the floor. Try it. I’ll bet it now gives a different figure.
You are assuming that you already know that the scale is only 95% accurate. Also you are also assuming that scale has a linear inaccuracy. It could easily be only 80% accurate at lower weights and be 95% accurate at higher weights.
No, I was assuming that the scale was in fact off by 95% and showing how you could determine that without any knowledge of that fact. You can do the same math with any assumption around degree of error, and it requires no knowledge of the degree of error ahead of time.
If you pencil out the algebra, you can see it using only variables and no numbers at all. It’s a simple proof.
Yes, I thought about linearity problem - you could address that by placing some arbitrary amount on the scale (say 5 20 pound weights), and then weigh the bucket by adding it to the stack. You could then use that weight amount instead of the direct measurement of the bucket (you could then directly weigh the bucket and see if there is a linearity problem, if you felt liek it). Honestly, though - unless the non-linearity is of an extraordinary magnitude, it should make little practical difference, largely due to the fact the bucket is a very small amount of the total weight.
Just like to point out one (probably obvious) point: time of day can make a difference. If you weigh yourself at home first thing in the morning after using the toilet, and then weight yourself at the gym later in the day after eating and drinking, the difference can be several pounds.
And, Picunurse, this means that my mother was wrong when she taught me “a pint’s a pound the world around.” I’m crushed. (Looking it up, I find that 16 fluid ounces of water is not the same weight as 16 ounces avoirdupois.)
Roddy
In practice in the biz, a balance is a laboratory-grade electronic instrument characterized by high resolution relative to the capacity. You’ll sometimes hear ‘top loading balance’ and ‘analytical balance’ when referring to these things but usually just ‘balance’ is used.
At my company, we (somewhat arbitrarily) call most devices over 50,000 graduations a balance which means things like tighter tolerances and more accurate calibration weight classes. For reference, the current state of the art is over 20 million grads and 2 million is ho-hum ordinary (say 210 x 0.0001 grams). The word certainly wouldn’t be applied to a doctor’s type scale except to describe its operation.
Of course, the term originated from historical devices of many kinds that did use fulcrums and knife edge pivots to balance loads against some known weight but there’s a pretty clear distinction. A triple beam scale, like the doctor’s scale, does rely on balance but they really aren’t called that outside of dictionaries and Wikipedia.
I would trust the home digital unit over the gym unit because the more scales are used the more they have a tendency to go out of whack and the mechanical balance units need to be calibrated regularly to be accurate if used heavily. Chances are that the gym unit is not accurate.
Agreed, but I think (please correct me if I’m wrong) that digital scales are neither spring or strain. Personally, I would go with the digital scales as being the more accurate of the two described in the OP.
The scales, balances and force measurement biz? Service, sales, rentals, onsite inspections. We’re an ISO17025 Accredited lab if that means anything to you.
If you were to walk into our office and ask for a balance with 300lb capacity and a height gauge, it would take a second to understand what you were asking for.
“You mean a doctor’s scale with a poise weight?”
To be clear, I think the term balance for beam/mechanical devices is probably dictionary-correct but increasingly obsolete (along with the devices themselves) in day to day usage and just plain wrong in industry. Load cells and A/D circuits have become really cheap.
We have furnished instruments to many interesting institutions such as eating disorder treatment facilities and pharma companies’ R&D for weighing humans and other large test animals. The idea of a mechanical scale for these applications is, frankly, laughable. Doctor’s scales are terribly unreliable and everyone knows it.
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Agreed, but I think (please correct me if I’m wrong) that digital scales are neither spring or strain. Personally, I would go with the digital scales as being the more accurate of the two described in the OP.
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Digital scales are typically strain gauge-based but they lose their linearity someplace between 20,000-75,000 graduations, depending on instrumentation and construction. This is where balances step in.
I agree that the digital bathroom scale is the one to use.
One advantage balances have over spring scales is that they do not need to stay in one place to remain accurate. A good balance will be accurate on the shores of the Dead Sea, on the top of K2, or the Moon. A spring scale depends on gravity staying the same to remain accurate.
Well, strictly speaking, that depends on whether you want to measure weight or mass. A spring scale will always tell you your weight, no matter where you are, and a balance will always tell you your mass, no matter where you are.
Digital home scales and cheap top-loading balances certainly use a strain gauge. But precision/analytical balances, such as those made by Mettler, use an electromagnetic force restoration (EMFR) load cell. No strain gauge is employed with EMFR… mass[sup]a[/sup] is measured using a coil, magnet, optical position sensor, and closed-loop control mechanism.
[sup]a[/sup][sub]Top-loading digital balances actually measure weight, not mass. But they will accurately report mass if they are properly calibrated and not moved.[/sub]
True. But there is confusion on how the terms “scale” and “balance” are commonly used. A digital/electronic top-loading unit (that uses a strain gauge load cell or EMFR load cell) measures weight and reports mass. Even though a balance beam is not used with this measurement device, a mass metrologist at NIST will refer to it as a balance, not a scale.
